1. Field of the Invention
The present invention relates to a color combining optical system used for a projection type image display apparatus such as a liquid crystal projector.
2. Related Background Art
As shown in FIG. 25, as the color combining optical system of a liquid crystal projector, a cross-dichroic prism XDP comprised of four right-angle prisms 61, 62, 63, and 64, with dichroic layers DM1 and DM2 having different reflection wavelength regions being made to cross each other within each prism, is widely used.
In the cross-dichroic prism, if the four right-angle prisms are not accurately polished, the dichroic layers DM1 and DM2 are crimped at a vertex of the right-angle prism. As a consequence, a projection image on a screen (not shown) becomes a double image, resulting in a considerable decrease in resolution.
In addition, in order to keep a high resolution on the screen, the four right-angle prisms 61, 62, 63, and 64 must be joined to each other without causing any level difference between the joining surfaces. This requires close attention in joining operation.
Defects such as so-called flaws and cracks are not allowed on the right-angle ridge portions of the right-angle prisms. If a ridge portion is wide, a crossing portion of a cross-prism becomes a vertical streak and is projected on the screen.
As described above, a conventional cross-dichroic prism is difficult to process and join, resulting in difficulty in attaining a reduction in manufacturing cost.
To solve the above problem in a cross-dichroic prism, a technique of applying a color separation prism constituted by three prisms and used for a video camera or the like to a liquid crystal projector has been proposed in Japanese Patent Application Laid-Open No. 10-104763.
The prism shape of the color separation prism proposed in the above reference is not optimized to minimize the optical path length of the prism. For this reason, the prism requires a prism optical path length about twice that of a cross-dichroic prism. In addition, since there is no description about the material for the prism and its refractive index in the above reference, it is not clear whether the optical path length at the time of air conversion is decreased by increasing the refractive index of the prism.
According to the arrangement proposed in the above reference, since dichroic films do not cross each other in the prism, this prism can be easily manufactured as compared with a cross-dichroic prism. However, since the prism optical path is long, the back focus of a projection lens needs to be long as compared with a case wherein a cross-dichroic prism is used. As a consequence, the projection lens becomes large in size and high in cost. With regard to the performance of the projection lens, the chromatic aberration of magnification increases.
In some liquid crystal projectors, in order to attain a reduction in size of a color combining dichroic prism, i.e., a reduction in overall size of the projector, a lens group having a positive refractive power is arranged between the incident surface of the color combining dichroic prism and a liquid crystal image display element to focus a light beam passing through the liquid crystal image display element and incident on the color combining dichroic prism regardless of whether a cross-dichroic prism or a dichroic prism other than the cross-dichroic prism is used. This arrangement allows a reduction in effective aperture of the color combining dichroic prism on the exit side, thus attaining reductions in sizes of the color combining dichroic prism and projection lens.
If, however, a light beam in a wavelength region that makes the light beam be reflected by the dichroic film in the color combining dichroic prism upon incidence is converged by the lens group, since the dichroic film is inclined with respect to the incident optical axis of the light beam, the incident angle of the light beam with respect to the dichroic film varies depending on the incident position on the dichroic film.
In addition, the reflection characteristics of the dichroic film depend on the incident angle of reflected light. If, therefore, the incident angle of a light beam with respect to the dichroic film varies depending on the incident position, the color-combined (i.e., projected) image suffers brightness unevenness or color unevenness.